A storm brings rain to California today — here’s what to expect
An unusual storm system is approaching the California coast, and its oscillating motion will determine how much rain the Bay Area will see on Wednesday, with the chances of heavy rain increasing as the day goes on.
The storm rotating offshore split into two eyes on Tuesday in an example of the Fujihara effect, and is expected to move closer to the coast over the next two days. Its front, pictured above, will streamline a series of rain bands across Northern California starting Wednesday afternoon. Each band will be able to produce heavier showers, but the precise nature of this meandering system could lead to a range of potential impacts for Bay Area residents.
Rainfall range
Due to the variable nature of storm strength and track, a wide range of rainfall totals are possible.
Rain chances will steadily increase after 1 p.m., with little or no rain across the Bay Area and Sacramento Valley into the evening. The difference between drizzle and more widespread rain will come down to how well the storm is able to maintain its consistency. If it does not consolidate, rainfall totals will be lower. Weather models indicate a tenth to a half-inch of rain across the Bay Area.
However, models ran up against rainfall forecasts on Tuesday as the storm split into two rotating areas of low pressure. For Wednesday’s rain to reach its full potential, the storm will have to overcome a few hurdles: It will have to absorb a lot of moisture and maintain its core, so its front can deliver heavy showers to the region. The system will also have to continue traveling towards the coast. If the storm oscillates a little longer over open water, rain won’t reach San Francisco and Oakland until Wednesday evening.
Shower activity is then expected to shift south Wednesday evening into Thursday morning, meaning residents from San Luis Obispo to Los Angeles will likely get anywhere from a quarter inch to an inch of rain. The highest totals in Southern California are likely to be concentrated on the leeward slopes of the Santa Monica and Santa Barbara mountain ranges, along with the San Gabriel, Santa Ana and San Bernardino mountains.
Right now, the most likely impacts are in the medium range for rainfall, which could range from a quarter to three-tenths of an inch in Northern California and up to an inch of rain in Southern California. Rain is likely to continue overnight and into Thursday and Friday, so plan to focus on the roads during your morning commute over the next couple of days.
Wednesday collapse
San Francisco: Residents can expect a mix of sunshine and light drizzle to help kick off the weather throughout the day, with cloudier conditions developing through the morning.
Look for intermittent rain chances in the afternoon and evening. Occasionally heavy rain is possible due to water and in the neighborhoods south and west of Sutro Tower. Daytime highs will remain around 60 degrees, while nighttime temperatures will only drop into the upper 50s.
East Bay: Mostly cloudy skies for residents in Alameda and Contra Costa counties this morning. Daytime highs will reach the mid 60s, while intermittent showers will occur in the afternoon.
The heavy rains are scheduled to reach the Gulf Shore and south-facing foothills in the Tri-Valley area, including Walnut Creek and Dublin. Rain will continue throughout the evening with overnight temperatures dropping to the 60 degree mark.
North Gulf: Residents in Marin, Sonoma, Napa and Solano counties will want to keep a raincoat on hand. The forecast calls for intermittent rain starting in the afternoon, with heavy rain possible on south-facing hillsides in areas like Novato, San Rafael and Santa Rosa. There is a slight chance of a thunderstorm or two developing along the Sonoma County coast and in the Napa Valley area during the evening hours.
Temperatures across the area will range from the mid 60s during the day to the upper 50s after sunset.
Pacific Coast and Peninsula: Residents along Highway 1 can expect good rain throughout the day, with morning drizzle developing into afternoon rain. Some of this rain is likely to fall intermittently across the peninsula during the evening. Temperatures will remain mostly in the lower 60s during the day and only drop into the upper 50s after dark, with warm and humid rain continuing through the night.
South Bay and Santa Cruz: A steady stream of drizzle is available for residents along Monterey Bay and morning commuters along the Highway 101 corridor from Gilroy to Morgan Hills. Skies will gradually darken across the Santa Cruz Mountains and Santa Clara Valley in the afternoon, as drizzle develops into rain and spreads inland.
Look for an increasing chance of showers throughout most of the afternoon and evening, followed by light drizzle overnight. Moderate air in the sky will result in daytime highs in the mid 60s and nighttime lows in the 60 degree mark.
Contact Gerry Diaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave
